Resonance resulting from torque pulse exerted on a rotary member will amplify noises and vibrations of machineries. Moreover, resonance will deteriorate durability of the machineries. For example, a crankshaft of an internal combustion engine is adapted to convert a linear reciprocating motion achieved by explosions in cylinders into rotation. Therefore, torque pulse appears inevitably on the crankshaft. For this reason, the crank shaft, an input shaft and a drive shaft of a transmission, a rotary member integrated with those shafts etc. are subject to torsional vibrations. In order to absorb or attenuate the torsional vibrations, a dynamic damper may be arranged in the rotary member. An example of the dynamic damper of this kind is disclosed in Japanese Patent Laid-Open No. 2000-18329. According to the teachings of Japanese Patent Laid-Open No. 2000-18329, a damper mass having a predetermined weight is held within a housing formed in a flywheel, and an ellipsoidal rolling guide surface on which the damper mass rolls is formed on radially outer portion of an inner face of the housing.
Another example is disclosed in Japanese Patent Laid-Open No. 6-58373. The flywheel taught by Japanese Patent Laid-Open No. 6-58373 comprises a rolling chamber formed in the flywheel, and a damper mass held in the rolling chamber. The damper mass is rolled in the rolling chamber by the torque pulse exerted on the flywheel.
In addition, Japanese Patent Laid-Open No. 11-82633 discloses a damper in which a pendulum assembly is fitted into a cylindrical recess formed on a surface of a flywheel. The pendulum assembly comprises a bottomed cylindrical casing whose center axis is parallel to a rotation axis of the flywheel, and a pendulum held by a bearing in a manner to pivot around the center axis of the casing. A weighted center of the pendulum is offset from the center axis of the casing.
By forming the rolling guide surface into an ellipsoidal shape as taught by Japanese Patent Laid-Open No. 2000-18329, a weighted center of the damper mass is allowed to trace an ellipsoidal hypocycloid. The damper thus structured can absorb or attenuate the torsional vibrations of the flywheel irrespective of magnitude of the torsional vibrations. However, in order to roll the dumper mass in a manner to trace the ellipsoidal hypocycloid by the weighted center as taught by Japanese Patent Laid-Open No. 2000-18329, it is necessary to form the rolling guide surface into the ellipsoidal shape. Therefore, a complex working is required to form the ellipsoidal guide surface on the housing in comparison with forming a curved face whose curvature is constant. Thus, the damper of by Japanese Patent Laid-Open No. 2000-18329 has to be improved.
Specifically, the flywheel taught by Japanese Patent Laid-Open No. 6-58373 is structured to absorb or attenuate the torsional vibrations resulting from torque pulse exerted thereon by a displacement of the weighted center of the damper mass, that is, by a pendulum motion of the damper mass. Meanwhile, the damper taught by Japanese Patent Laid-Open No. 11-82633 is formed by suspending the pendulum assembly comprising a mass pivotally from the rotation center of the casing through the bearing. Therefore, the pendulum assembly is oscillated by the torque pulse thereby absorbing or attenuating the torsional vibrations.